The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Substrate processing systems may be used to etch film on a substrate such as a semiconductor wafer. The substrate processing systems typically include a processing chamber, a gas distribution device, and a substrate support. During processing, the substrate is arranged on the substrate support. Different gas mixtures may be introduced into the processing chamber. Radio frequency (RF) plasma may be used to activate chemical reactions.
In some situations, it is important to detect trace levels of molecular oxygen (O2) in the vicinity of an upper surface of the substrate. For example, some photoresist strip or surface modification processes are very sensitive to oxidation of underlying films. Current approaches for measuring trace levels of molecular oxygen in a vacuum chamber typically use a residual gas analyzer (RGA). The operation of the RGA is accomplished using an open ion source (OIS) or a closed-ion-source (CIS) that ionizes gas and detects mass based on quadrupole spectroscopy. RGA typically has a sensitivity that is less than 1 part per million (ppm). OIS detection systems are directly mounted to the processing chamber and have a maximum operating pressure of 10−4 Torr. CIS detection systems typically require a differential pumping system and therefore have reduced sensitivity. Furthermore, the sensitivity of the RGA decays over time due to gas corrosion (˜1 year with a molecular hydrogen (H2) process) and then costly replacement is required.